Added the --remove-non-retargetable option.

Added the --remove-non-retargetable option to the
remove-external-types-usage command.
retarget-assembly += remove-external-types-usage

The retarget-assembly command now also invokes
remove-external-types-usage.
Using ReferenceSearchingMetadataResolver

Using ReferenceSearchingMetadataResolver in RetargetAssemblyProcessor.
Tracing ReferenceSearchingMetadataResolver.
Updated readme.
Added the make-portable-skeleton command.

Added the make-portable-skeleton command. It's equivalent to
make-skeleton + remove-pinvoke-methods + retarget-assembly. This command
removes all resources, code, private members, private types and P/Invoke
methods then retargets the assembly to the specified (preferably
portable) .Net Framework profile and removes all members that reference
types outside the assembly, framework assembly set or retargetable
assembly references.